Up to now, as disclosed in Patent Literature 1, WAVE (Wireless Access in Vehicular Environments) has been known as a communication standard used for an intelligent transport system. In the WAVE, a vehicle wireless communication device and a roadside device communicate with each other with the use of two kinds of channels, that is, a control channel and a service channel.
A service channel is a channel used for transmitting and receiving information for executing a service (hereinafter, service execution information). There are multiple types of services, and the service channels also have multiple channels having different frequencies from each other. Each service is associated with one of the multiple service channels.
The control channel is a channel used for transmitting various kinds of information (hereinafter, service start information) required to be notified in order that the roadside device starts a communication with the vehicle wireless communication device with the use of the service channel. In order to start the communication using the service channel, there is a need to set a channel (hereinafter referred to as a reception channel) to be used for reception by the vehicle wireless communication device as one of the service channels. For that reason, the service start information includes information that defines the service channel to be set for the reception channel.
In the WAVE, a time period is divided into a control channel time frame and a service channel time frame. The control channel time frame is a time frame in which the vehicle wireless communication device uses the reception channel as the control channel unless a command to change the service channel time frame is notified by the service start information. On the other hand, the service channel time frame is a time frame for periodically setting the reception channel to the service channel while a communication using the service channel is being performed.
Until the vehicle wireless communication device receives the service start information, the vehicle wireless communication device sets the reception channel to the control channel in not only the control channel time frame but also in the service channel time frame. On the other hand, in the case of receiving the service start information, the vehicle wireless communication device sets the reception channel to the service channel from the first service channel time frame after receiving the service start information. Thereafter, until the communication for executing the service is completed normally, the vehicle wireless communication device sets the communication channel as the service channel designated by the service start information in the service channel time frame, and sets the communication channel as the control channel in the control channel time frame.
In addition, the service start information includes information indicating a priority of the service. Suppose that certain service start information has been received while the reception channel is set as the control channel and thereafter another service start information designating another service has been received before a start of the service channel time frame. In this case, out of the services designated by the service start information received during the control channel time frame, the service high in the priority is executed.